Journal of Radioana!yticat and Nuclear Chemistry, Articles, Vo!. 160, No. 1 (1992) 159-168 IN VIVO MEASUREMENTS OF NITROGEN, HYDROGEN, AND CARBON IN GENETICALLY OBESE AND LEAN PIGS K. J. ELLIS, R. J. SHYPAILO, H.-P. SHENG, W. G. POND USDA/ARS Children's Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine, Houstor~ Texas (USA) (Received December 13, 1991) Characteristic gamma-rays are emitted promptly by elements during exposure to neutrons. These emissions enable a radioanalytical analysis of the body's composition of protein (nitrogen), water (hydrogen), and fat (carbon). We have used this method in vivo to determine the body composition of obese and lean pigs (10 to 20 kg body wt) fed an altered cholesterol diet. Xntroduction Because the measurement of body protein mass has provided important information about the nutritional status of adult patients, 1 we now believe such measurements may also prove useful in the diagnosis and treatment of children, newborn infants, or possibly premature infants. In addition, it may be very useful to know the changes in protein and fat levels that occur in growing animals maintained under different dietary regimens. As part of an animal study that focused on the impact of cholesterol deprivation in early life on later cholesterol metabolism, we wanted to develop nuclear techniques with which to examine body composition. Animal studies have indicated that early nutrition can affect later cholesterol metabolism, and possibly body composition. This general topic has practical importance, for example~ in the feeding of a low-cholesterol formula to an infant whose genetic background differs from average with respect to lipid metabolism. Although evidence in humans is equivocal, studies in animals have emphasized the importance of critical periods during development when changes in diet can have lasting consequences. The pig has been established as a suitable animal model for studies of the cardiovascular system and atherosclerosis,213 and for determining nutritional relationships with atherogenesis4. We were particularly interested in studying the relationships between genetic background~ response to dietary Else~,,ier Sequoia S. A, Lausanne Akaddmiai Kiad6, Budapest